read-cache*.h: move declarations for read-cache.c functions from cache.h

For the functions defined in read-cache.c, move their declarations from
cache.h to a new header, read-cache-ll.h.  Also move some related inline
functions from cache.h to read-cache.h.  The purpose of the
read-cache-ll.h/read-cache.h split is that about 70% of the sites don't
need the inline functions and the extra headers they include.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rev-parse.c

@@ -18,6 +18,7 @@
 #include "object-name.h"
 #include "parse-options.h"
 #include "diff.h"
+#include "read-cache-ll.h"
 #include "revision.h"
 #include "setup.h"
 #include "split-index.h"